This invention relates to offshore structures and, more particularly, to tension leg platforms (TLP) for exploitation of deep sea hydrocarbon reserves.
Mooring elements, or tethers, on tension leg platforms are anchored to the seabed. They usually consist of steel pipes and are kept in tension by the buoyancy of the platform.
With the gradual depletion of onshore and shallow sub sea subterranean hydrocarbon reservoirs, the search for additional petroleum reserves is being extended into deeper and deeper waters. As such deeper reservoirs are discovered, increasingly complex and sophisticated production systems are being developed. It is projected that soon, off-shore exploration and production facilities will be required for probing depths of 1500 m or more.
One way of reaching these depths is by using Tension Leg Patforms. A TLP comprises a semi-submersible-type floating platform anchored to foundations on the sea bed through members or mooring lines called tension legs or tethers. The tension legs are maintained in tension at all times by ensuring that the buoyancy of the TLP exceeds its operating weight under all environmental conditions. The TLP is compliantly restrained by this mooring system against lateral offset allowing limited surge, sway and yaw. Motions in the vertical direction of heave, pitch and roll are stiffly restrained by the tension legs.
External flotation systems can be attached to the legs, but their long-term reliability is questionable. Furthermore, added buoyancy of this type causes an increase in the hydrodynamic forces on the leg structure.
TLPs based on today's technology are considered competitive down to 1,000-1,500 M. Beyond this depth, the tether system becomes increasingly heavy, requiring an increased platform size to carry the tether weight. This results in a larger platform, which has a significant impact on the overall cost.
For a TLP at 3,000 m, a conventional tether system (one thickness, one diameter) represents a weight almost equal to the payload. In previous designs, it has been proposed to reduce the wall thickness at the top to reduce the weight penalty. A solution to avoid these disadvantages related to the TLP is to modify the tether system to reduce the need for increased hull size. The industry has devoted a considerable effort to develop tether systems based on various designs. Filling tether pipes with low-density material, pressurizing the interior to increase the hydrostatic capacity, and replacing the steel tether pipes with composites are examples of these efforts.
Another solution can be found in NO 1997 3044, showing a design used for depths down to 700 m, built by pipe sections with a diameter between 0.5 and 1.2 m. The overall buoyancy of the tension leg is meant to be more or less neutral. This is achieved by adding an additional floating body at the top of the pipe.
NO 1997 3045 shows a welding connection on a tension leg. The publication shows two pipes having different diameters and wall thicknesses welded together.
GB 2 081 659 A shows a floating platform mooring system for use in exploiting sub sea oil shoals, and includes a platform structure and an array of vertical tubular anchoring lines connected to the upright of the platform structure and to anchoring blocks on the sea bed. The patent shows anchoring lines consisting of a steel tube having resistance to yield stresses and having upper and lower sections. The upper section is a steel rod with a flexural stiffness which decreases from its point of connection to the upright. The lower section of the anchoring line has a hollow configuration and is fixed to an anchoring block in order to achieve an optimum exploitation of the structural material.
However, the patent does not address the problems relating to the weight and pressure resistance of deep sea tension legs.